1. Field of the Invention
The invention relates to the field of thermal detectors, especially so-called BLIP (background limited performance) high performance thermal detectors. These thermal detectors have a sensitivity which is theoretically limited by the thermal conductivity noise between the detector and the external environment. This noise arises from the fluctuation in the number of photons coming from the external environment and arriving on the surface of the thermal detector.
2. Discussion of the Background
Thermal detectors are detectors which measure, directly or indirectly, the heat produced by the incident radiation to be detected when it arrives on the surface of the detector, which heat is transformed into a temperature rise at the detector surface. Preferably, the spectral sensitivity range of the thermal detectors is situated in the infrared.
Generally, the scene observed by a thermal detector is xe2x80x9cseenxe2x80x9d by the latter under a certain viewing angle called viewing angle of the scene. When the thermal detector is part of an optical architecture, the viewing angle of the scene is determined by the aperture of the entrance optics of the optical architecture. In order to limit the noise received by the thermal detector, it is beneficial to limit the viewing angle of the thermal detector to a given angle, preferably equal to the viewing angle of the scene. Thus, without loss of signal, the noise received by the thermal detector will be decreased.
Thermal detectors are subdivided into two families, cooled thermal detectors and uncooled thermal detectors. A cooled thermal detector generally comprises a cold shield. This cold shield is pierced by an aperture which limits the viewing angle of the detector to the viewing angle of the scene. This is because, the cold shield is at low temperature, typically about 77 kelvin. Thus the thermal emission of the cold shield is negligible compared to the thermal emission of the observed scene since the latter is much hotter, typically 300 kelvin. The cold shield provides a marked improvement in the performance of a thermal detector by limiting its viewing angle. Similarly, the aperture of the cold shield may contain a spectral filter cooled like the cold shield. Thus, the spectral sensitivity range of the thermal detector may be reduced to certain windows corresponding advantageously to the windows of atmospheric transparency.
Since the uncooled thermal detectors do not have a cold shield, this solution is difficult to apply without also cooling the detectors, which would decrease their advantage. Consequently, the invention is applicable most particularly to uncooled detectors.
According to the prior art, several types of uncooled thermal detectors have been proposed. The majority of them are detectors with microbolometers, i.e. matrix detectors, the pixels of which are made of a material whose resistance varies according to the temperature. The pixels have one or more layers depending on the type of thermal detector. However, one property common to these different types of pixel is that of having an absorbent layer which absorbs the majority of the incident radiation even when the latter has an almost horizontal incidence, i.e. making a large angle with the normal to the detector surface.
The invention makes it possible, as in the case of cooled thermal detectors, to limit the viewing angle of the detector, preferably to the viewing angle of the scene. For this, each pixel is individually provided with a biperiodic structure, the pitch of which is smaller than the mean wavelength of the spectral sensitivity range of the detector and which has the effect of limiting the viewing angle of the thermal detector. This limitation is obtained by the transition of the pixel from a rather absorbing state to a rather reflecting state depending on the angle made with the normal to the surface of the detector by the incident radiation upon its arrival on the surface of the thermal detector. The choice of parameters for this biperiodic structure makes it possible to limit the viewing angle of the detector to the value required by the particular application envisaged. This biperiodic structure is a biperiodic grating of elementary blocks, the angular selectivity of which is high enough to allow the usual conditions of the envisaged applications to be satisfied.
According to the invention, an uncooled thermal matrix detector is provided having a given spectral sensitivity range and being formed of pixels which are thermally insulated from each other, each pixel comprising an absorbent element which is thermosensitive in the spectral sensitivity range, characterized in that on the face, turned toward the incident radiation to be detected, of each thermosensitive absorbing element is placed a biperiodic grating of elementary blocks limiting the viewing angle of the detector, and in that the pitch of the grating is less than the mean wavelength of the spectral sensitivity range.